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A permutation of a subset of k elements from set of n elements is ordered sequence of k distinct elements from this set. The number of these permutations is given by formula n_P_k = n! / (n-k)!. Examples. A permutation of 2 elements from set of 3 elements: a, b, c. ab, ac, ba, bc, ca, cb. 3_P_2 = 3! / (3-2)! = 1⋅2⋅3 / 1! = 6. A permutation of 3 elements from 4 elements: a, b, c, d. abc, abd, acb, acd, adb, adc, bac, bad, bcd, bda, bdc, cab, cad, cbd, cda, cdb, dab, dac, dba, dbc, dca, dcb. 4_P_3 = 4! / (4-3)! = 1⋅2⋅3⋅4 / 1! = 24. A permutation of 2 elements from 4 elements a, b, c, d: ab, ac, ad, ba, bc, bd, ca, cb, cd, da, db, dc. 4_P_2 = 4! / (4-2)! = 1⋅2⋅3⋅4 / 2! = 12.